1. Technical Field of the Invention
This invention relates to machine tools, and more specifically, to machine tools that include means for automatically controlling their operation.
2. Description of the Prior Art
In the fabrication of numerous commercial products, oftentimes it is necessary to drill precise holes in the product. For example, in fabricating multilayer printed circuit boards, the drilled holes must be of sufficient quality to permit the plating of a thin layer of copper about the hole periphery. Any projections or rough edges around the hole periphery may result in inadequate plating thickness or plating uniformity. These inadequacies, in turn, could lead to failure of the printed circuit board and the equipment of which it is a part.
Drilling operations which are to be carried out on items to be mass produced are frequently undertaken by multiple spindle drilling machines having automatic workpiece advance mechanisms built into them. As the workpiece is fed beneath the drill spindles, the spindles are lowered and raised cyclically so that the drilling operation can be implemented in as nearly an automatic process as possible. One problem, however, in undertaking drilling operations in this manner is that drill bits can lose their sharpness or in fact become broken before an operator can detect such a condition. As a result several workpieces could be ruined.
In a paper entitled "Metal-Cutting Forces and Power Requirements for Machine Tools" by O. W. Boston appearing in Automotive and Aviation Industries, Volume 86, May, 1942 at page 32, there is an indication that a dynamometer may be used on a lathe or planer in such a manner as to determine the tangential cutting force alone or the three components (tangential, longitudinal, and radial) of the cutting force.
A generalized description entitled "Relationship of Forces and Power Consumption to Machine Variables" appears in Tool Engineers Handbook 2nd Edition published by McGraw-Hill Book Company, Copyright 1959 at pages 18-43 and 18-44. In particular, it is noted that power consumption in a machining operation is roughly proportional to the cutting speed, since the rate at which metal is removed is proportional to that speed. It is further noted that the effect of cutting speed on certain tool forces depends upon a number of factors including the tool-work combination, tool geometry, speed range involved, feed, depth of cut, and type of machining operation. Unfortunately, both of the aforementioned references are devoid of any suggestion of a method or apparatus for determining the wear of a drill bit in real time.
A reference which addresses the problem of monitoring rotating parts is U.S. Pat. No. 3,961,184 issued June 1, 1976 to J. Schurrer. Schurrer relates to a device for making photoelectric measurements of moving parts. The moving part is provided with contrast markings thereon and a source of light is directed onto the contrast marks. Light reflected from the moving part is picked up by a receiver and the periodic sensing of this light can be translated into a measurement of rotational speed of the moving part. While Schurrer represents an advance in the art of monitoring moving parts, there is no suggestion of a solution to the problem of detecting when a drill bit either loses sharpness or becomes broken.
A more recent advance in automatic monitoring systems is disclosed in R. C. Miller et al, U.S. Pat. No. 4,023,044 issued May 10, 1977. Miller et al disclose a monitoring system comprised of a transducer mounted in association with a machine tool for producing a signal in response to mechanical impulses developed by operation of the machine tool. The signal is detected and applied to the input of means for generating a test function. This test function is a function of both the time and the amplitude of the transducer signal. Means are provided for producing a reference function proportional to the test function evaluated during normal operation of the machine tool and for comparing this reference function with the test function evaluated during subsequent operations. A comparator generates an output signal for controlling operation of the machine tool when the test function and the reference function differ by more than a predetermined amount. Even with this advance the Miller et al reference falls short of the mark for providing a solution to the problem of determining the wear on a machine tool, such as a drill bit, in real time.
In view of the foregoing, is should be apparent that a number of problems have not been addressed by the prior art. Specifically, none of the references deals with a way to monitor the quality of a drill bit and a drilled hole in real time. Moreover, the problem associated with the detection of broken drill bits has not been considered, even remotely. Still other problems requiring solution are those related to providing an indication when the drill bit quality dips below a standard so that the drill bit can be changed; ways to limit the amount of energy expended in a drilling operation; methods to ascertain the drillability of various types of material; and methods to evaluate the quality of drills supplied by different manufacturers.